Certain alternative enzymatic reactions lead to production of dTTP, one of the deoxynucleotides required to be present in the cell for DNA synthesis, a prerequisite for cellular division. These enzyme reactions are prominent in embryonic and regenerating liver as well as a variety of hepatomes. In vitro assays of these enzymes: deoxythymidine kinase, dioxythymidylate kinase, deoxythymidylate synthetase, deoxycytidylate deaminase and ribonucleotide reductase will be performed on embryo lysates to determine which enzyme activities are present, how they change during the earliest stages of mouse development, and how these enzymes may be feedback controlled by dTTP and ATP. In vitro incubations of embryos in radioactive nucleosides and subsequent identification of intracellular radioactive nucleotides will give clues to which of the pathways predominate in vivo. Embryos can be fused to 2-15 times their normal size in cell number initially, but if allowed to continue developent, are normal-sized at birth. Certain hypotheses will be tested in biochemical terms to explain the regulation of size which must be exerted for this to occur. Changes will be sought in activities of enzymes producing dTTP and in the embryo's ability to take up and incorporate precursors into DNA, RNA and protein.